Hexavalent chromium reduction by an actinomycete, arthrobacter crystallopoietes ES 32.

Environmental contamination by hexavalent chromium, Cr(VI), presents a serious public health problem. This study assessed the reduction of Cr(VI) by intact cells and a cell-free extract (CFE) of an actinomycete, Arthrobacter crystallopoietes (strain ES 32), isolated from soil contaminated with dichromate. Both intact cells and CFE of A. crystallopoietes, displayed substantial reduction of Cr(VI). Intact cells reduced about 90% of the Cr(VI) added within 12 h and Cr(VI) was almost completely reduced after 24 h. The KM and Vmax of Cr(VI) bioreduction by intact cells were 2.61 microM and 0.0142 micromol/min/mg protein, respectively. Cell-free chromate reductase of the A. crystallopoietes (ES 32) reduced hexavalent chromium at a KM of 1.78 microM and a Vmax of 0.096 micromol/min/mg protein. The rate constant (k) of chromate reduction was inversely related to Cr(VI) concentration and the half-life (t1/2) of Cr(VI) reduction increased with increasing concentration. A. crystallopoietes produced a periplasmic chromate reductase that was stimulated by NADH. Results indicate that A. crystallopoietes ES 32 can be used to detoxify Cr(VI) in polluted sites, particularly in stressed environments.

Chromate reduction by chromium-resistant bacteria isolated from soils contaminated with dichromate.

Extensive use of hexavalent chromium [Cr(VI)] in various industrial applications has caused substantial environmental contamination. Chromium-resistant bacteria isolated from soils can be used to remove toxic Cr(VI) from contaminated environments. This study was conducted to isolate chromium-resistant bacteria from soils contaminated with dichromate and describes the effects of some environmental factors such as pH, temperature, and time on Cr(VI) reduction and resistance. We found that chromium-resistant bacteria can tolerate 2500 mg L(-1) Cr(VI), but most of the isolates tolerated and reduced Cr(VI) at concentrations lower than 1500 mg L(-1). Chromate reduction activity of whole cells was detected in five isolates. Most of these isolates belong to the genus Bacillus as identified by the 16S rRNA gene sequencing. Maximal Cr(VI) reduction was observed at the optimum pH (7.0-9.0) and temperature (30 degrees C) of growth. One bacterial isolate (Bacillus sp. ES 29) was able to aerobically reduce 90% of Cr(VI) in six hours. The Cr(VI) reduction activity of the whole cells of five isolates had a K(M) of 0.271 (2.61 mM) to 1.51 mg L(-1) (14.50 mM) and a V(max) of 88.4 (14.17 nmol min(-1)) to 489 mg L9-1) h(-1) (78.36 nmol min(-1)). Our consortia and monocultures of these isolates can be useful for Cr(VI) detoxification at low and high concentrations in Cr(VI)-contaminated environments and under a wide range of environmental conditions.

In vitro reduction of hexavalent chromium by a cell-free extract of Bacillus sp. ES 29 stimulated by Cu2+.

Chromium-resistant bacteria (CRB) isolated from soils can be used to reduce toxic Cr(VI) from contaminated environments. This study assessed in vitro reduction of hexavalent Cr using a cell-free extract (CFE) of CRB isolated from soil contaminated with dichromate. One isolate, ES 29, that substantially reduced Cr(VI) was identified as a Bacillus species by 16S rRNA gene-sequence homology. The isolate reduced Cr(VI) under aerobic conditions, using NADH as an electron donor and produced a soluble Cr(VI)-reducing enzyme stimulated by copper (Cu2+). The CFE of the bacterial isolate reduced 50% of Cr(VI) in 6 h. The Cr(VI)-reduction activity of the CFE had a Km of 7.09 microM and a Vmax of 0.171 micromol min(-1) mg(-1) protein. Mercury inhibited the enzyme, but not competitively, with a Vmax of 0.143 micromol min(-1) mg(-1) protein, a Km of 7.07 microM and a Ki of 1.58 microM. This study characterizes the enzymatic reduction of Cr(VI) by Bacillus sp. ES 29 which can be used for the bioremediation of chromate.

Bioremediation of selenium-contaminated sediments and water.

Selenium (Se) is a contaminant of agricultural irrigation-drainage water in the western United States, and the cause of wildlife deaths and grotesque deformities. Some approaches in reducing the toxic Se concentrations from contaminated sediments and water have been proposed, but most of these tend to be costly or ineffective. Bioremediation through microbial transformations of toxic Se species into nontoxic forms is being considered as an effective remedial alternative. The microbial reduction of toxic oxyanions of Se (SeO(4)(2-) and SeO(3)(2-)) into insoluble Se(0) or methylation of these species to dimethylselenide (DMSe) has been accepted as a potential bioremediation strategy for cleanup of Se-contaminated water and sediments. By conducting a series of laboratory, bench-scale and field studies, we have thoroughly investigated the remedial potential of these approaches. It was observed that microorganisms, particularly Enterobacter cloacea, are very active in reduction of Se oxyanions present in irrigation drainage water, into insoluble Se(0) and, by monitoring various environmental conditions and addition of organic amendments, the process could be stimulated manifold. Similarly, the process of biomethylation of Se in soil sediments and water was found active and highly dependent on specific carbon amendments (pectin and proteins), pH, temperature, moisture, aeration and activators (cofactors). Moreover, Se biomethylation was protein/peptide-limited rather than nitrogen-, amino acid- or carbon-limited. Crude casein and its components were equally stimulatory producing a >50-fold enhancement in DMSe yield. Methionine and methyl cobalamin stimulated DMSe production by Alternaria alternata, indicating that the coenzyme may mediate the transfer of a methyl group to the Se atom. An acute toxicity test involving inhalation of DMSe by rats revealed that DMSe is nontoxic. Experiments were scaled up from laboratory studies to field plots to verify the feasibility of this bioremediation approach. Based upon the promising results of these studies, a biotechnology prototype was developed which could be applicable for cleanup of polluted sediments and water throughout the western United States.

Coupled production and transport of selenium vapor in unsaturated soil: evaluation by experiments and numerical simulation.

Volatilization of selenium (Se) from soil to the atmosphere involves several sequential chemical reactions that form volatile Se species, followed by transport of the gaseous Se through the soil. This paper describes a numerical model that simulates the chemical and physical processes governing the production and transport of Se vapor in unsaturated soil. The model couples the four Se species involved in the production of Se vapor through chemical reactions, and allows each to migrate through the soil by advection, liquid or vapor diffusion depending on its affinity for the dissolved or vapor phase. The coupled transformations and transport of the four Se species, i.e., selenate, selenite, elemental and organic Se, and Se vapor, were calculated based on the Crank-Nicolson finite difference method. The model was used to analyze fluxes of Se vapor measured from a soil amended with inorganic Se in the form of selenate and covered with unamended clean soil of various thicknesses. Evolution of Se vapor from the soil was very fast, with measurable amounts of Se detected within 24 h. The peak of Se volatilization, detected at the 6th day, reached 3.31 Se microgram/day for the uncovered soil, but was reduced to near the detection limit (0.05 microgram/day) in the presence of a 8- or 16-cm clean soil cover. With two reaction rate coefficients fitted to the data, the model described Se volatilization very well. The estimated rate coefficient of Se methylation was unexpectedly high, with a value of 0.167/day. The net volatilization of Se, however, was severely inhibited by the fast demethylation, i.e., the reverse reaction which converted volatile Se species back into nonvolatile forms. As a result, Se vapor only penetrated a few centimeters in the soil. The demethylation rate coefficient, assessed by independent transport experiments using dimethyl selenide, was estimated as 186.8/day, corresponding to a half-life of only 5.3 min for Se vapor. Results of this study indicated that rapid demethylation of Se vapor during its diffusive transport through a soil is probably an important limiting factor in the volatilization of Se under natural conditions.

Speciation of selenium in plant water extracts by ion exchange chromatography-hydride generation atomic absorption spectrometry.

Determination of selenium (Se) speciation in plants is important in studying the bioavailability and toxicity of Se in Se-contaminated soil/sediment. In this study, we used an anion exchange resin (Dowex 1-10X) to separate Se into non-amino acid organic Se, Se-amino acids, selenite (Se [IV]) and selenate (Se [VI]) in a plant (Stanleya pinnata) extract. The hydride generation atomic absorption spectrometry (HGAAS) was used to determine concentrations of these Se compounds in plant extracts. Results showed that Se compounds can be quantitatively separated by the resin column. Recovery of five spiked standard Se compounds (trimethylselenonium ion (TMSe+), dimethylselenoxide (DMSeO), selenomethionine (Semet), Se [IV] and Se [VII]) in the plant extract ranged from 92.9 to 103%. Water extractable Se accounted for 60.4-72.6% of the total Se in the plant. Among the soluble Se compounds in the plant extract, Se-amino acids were 73-85.5%, Se [VI] ranged from 7.5 to 19.5% and non-amino acid organic Se was less than 7%. Se [IV] in most samples was below the detection limit (1 microg/g). This study showed that considerable amounts of the accumulated Se [VI] in the plant was metabolized to Se-amino acids during growth of the plant.

Perchlorate and nitrate reductase activity in the perchlorate-respiring bacterium perclace.

The perchlorate (ClO4(-))-respiring organism, strain perclace, can grow using nitrate (NO3(-)) as a terminal electron acceptor. In resting cell suspensions, NO(-) grown cells reduced ClO4(-), and ClO4(-) grown cells reduced NO3(-). Activity assays showed that nitrate reductase (NR) activity was 1.31 micromol min(-1) (mg protein)-1 in (ClO4)- grown cells, and perchlorate reductase (PR) activity was 4.24 micromol min(-1) (mg protein)(-1) in NO3(-) grown cells. PR activity was detected within the periplasmic space, with activities as high as 14 pmol min(-1) (mg protein)(-1). The NR had a pH optimum of 9.0 while the PR had an optimum of 8.0. This study suggests that separate terminal reductases are present in strain perclace to reduce NO3(-) and ClO4(-).

Effect of moisture and casein on demethylation of trimethylselenonium in soil.

Trimethylselenonium ion (TMSe+) is a major urinary selenium (Se) metabolite of animals. The effect of soil moisture and casein on the demethylation of TMSe+ were examined in three California soils [Tulare Basin (TL), Losthill (LH) and Five Points (FP) upland areas] treated with 20 microg/g TMSe+. Results showed that soil moisture positively affected the demethylation of TMSe+ to volatile dimethylselenide (DMSe). In the non-saline TL and FP soils, the highest demethylation rates were 0.169-0.454, 0.646-2.71, and 3.15-6.01 microg/g per day from a 10, 20, and 100% moisture soil, respectively. During 51 days of the experiment, approximately 75.4-80.2% of added TMSe+ was demethylated to volatile DMSe. In contrast, in a saline LH soil, the demethylation rate was < 0.2 microg/g per day, and only 7% of the added TMSe+ was demethylated to volatile DMSe. Addition of casein stimulated the demethylation of TMSe+ in a 10% moisture TL soil. Dissolved DMSe, dimethylselenoxide (DMSeO) and TMSe+, and adsorbed TMSe+ were found in the three experimental soils. Concentrations of dissolved and adsorbed TMSe+ were much higher in the LH soil than in the TL and FP soils. This study showed that demethylation of TMSe+ dominates in the FP and TL soils, and adsorption of TMSe+ is a more important process in the LH soil.

Accelerated volatilization rates of selenium from different soils.

Selenium (Se), an element found naturally in a variety of soils, can accumulate in drainage water of lands under intensive irrigation, even reaching levels that are toxic to mammals and birds. Volatilization of Se by soil microorganisms into dimethylselenide (DMSe) can be enhanced by certain soil amendments and, thus, be used as a soil remediation process. In an 8-wk laboratory study, five soils from California and one from Germany were spiked with 75SeO3(2-) (22.3 mg/kg Se). Two amino acids (DL-homocysteine and L-methionine), a carbohydrate (pectin), and a protein (zein) were tested as soil amendments. Gaseous 75Se emissions were trapped with activated carbon and measured in a gamma counter. Depending on soil type, the cumulative volatilization from the control flasks varied between 1.2% and 9.0% of applied 75Se. Both zein and L-methionine strongly increased volatilization (max. 43% of 75Se applied), whereas DL-homocysteine had a much smaller stimulating effect. Pectin showed a moderate effect, but enhanced Se volatilization rates were sustained much longer when compared to the zein amendment. Volatilization rates of Se followed a simple first-order reaction. Gaseous Se emission in the soils treated with L-methionine yielded an S-shaped curve, which fit a growth-modified first-order rate model. Although zein and L-methionine were the most favorable treatments enhancing Se volatilization, all six soils responded differently to the soil amendments.

Reduction of Selenium Oxyanions by Enterobacter cloacae SLD1a-1: Isolation and Growth of the Bacterium and Its Expulsion of Selenium Particles.

A facultative bacterium capable of removing the selenium (Se) oxyanions selenate (SeO(inf4)(sup2-)) and selenite (SeO(inf3)(sup2-)) from solution culture in flasks open to the atmosphere was isolated and studied with the goal of assessing its potential for use in bioremediation of seleniferous agricultural drainage water. Elemental Se (Se(sup0)) was confirmed as a product of the reaction. The organism, identified as Enterobacter cloacae and designated strain SLD1a-1 (ATCC 700258), removed from 61.5 to 94.5% of added SeO(inf4)(sup2-) (the primary species present in agricultural drainage water) at concentrations from 13 to 1,266 (mu)M. Equimolar amounts of nitrate (NO(inf3)(sup-)), which interferes with SeO(inf4)(sup2-) reduction in some organisms, did not influence the reaction in growth experiments but had a slight inhibitory effect in a washed-cell suspension. Washed-cell suspension experiments also showed that (i) SeO(inf3)(sup2-) is a transitory intermediate in reduction of SeO(inf4)(sup2-), being produced and rapidly reduced concomitantly; (ii) NO(inf3)(sup-) is also reduced concomitantly and at a much higher rate than SeO(inf4)(sup2-); and (iii) although enzymatic, reduction of either oxyanion does not appear to be an inducible process. Transmission electron microscopy revealed that precipitate particles are <0.1 (mu)m in diameter, and these particles were observed free in the medium. Evidence indicates that SLD1a-1 uses SeO(inf4)(sup2-) as an alternate electron acceptor and that the reaction occurs via a membrane-associated reductase(s) followed by rapid expulsion of the Se particles.

Influence of surfactants on pyrene desorption and degradation in soils.

Four surfactants were tested at five concentrations to determine their abilities to solubilize soil-adsorbed pyrene. Inoculation with pyrene degraders in the presence of the surfactant Witconol SN70 was the most effective treatment for pyrene mineralization (46 to 80%) under unsaturated conditions, but the surfactant inhibited the effectiveness of these inoculants in soil slurries.

Environmental biochemistry of chromium.

Chromium is a d-block transitional element with many industrial uses. It occurs naturally in various crustal materials and is discharged to the environment as industrial waste. Although it can occur in a number of oxidation states, only 3+ and 6+ are found in environmental systems. The environmental behavior of Cr is largely a function of its oxidation state. Hexavalent Cr compounds (mainly chromates and dichromates) are considered toxic to a variety of terrestrial and aquatic organisms and are mobile in soil/water systems, much more so than trivalent Cr compounds. This is largely because of differing chemical properties: Hexavalent Cr compounds are strong oxidizers and highly soluble, while trivalent Cr compounds tend to form relatively inert precipitates at near-neutral pH. The trivalent state is generally considered to be the stable form in equilibrium with most soil/water systems. A diagram of the Cr cycle in soils and water is given in Fig. 6 (Bartlett 1991). This illustration provides a summary of environmentally relevant reactions. Beginning with hexavalent Cr that is released into the environment as industrial waste, there are a number of possible fates, including pollution of soil and surface water and leaching into groundwater, where it may remain stable and, in turn, can be taken up by plants or animals, and adsorption/precipitation, involving soil colloids and/or organic matter. Herein lies much of the environmental concern associated with the hexavalent form. A portion of the Cr(VI) will be reduced to the trivalent form by inorganic electron donors, such as Fe2+ and S2-, or by bioprocesses involving organic matter. Following this conversion, Cr3+ can be expected to precipitate as oxides and hydroxides or to form complexes with numerous ligands. This fraction includes a vast majority of global Cr reserves. Soluble Cr3+ complexes, such as those formed with citrate, can undergo oxidation when they come in contact with manganese dioxide, thus reforming hexavalent Cr. In trace amounts, Cr is an essential component of animal nutrition, functioning mainly in glucose metabolism, and possibly in fat metabolism. While shown to be nonessential for plants, it is required by some microbes, possibly as a cofactor for specific enzyme systems. Bacteria with plasmid-conferred resistance to Cr(VI) have been isolated from water, soil, and sediments, and the resistance mechanisms have been somewhat characterized. One of the chief mechanisms is bioreduction of toxic Cr(VI) to the relatively nontoxic Cr(III). This has been shown to occur directly, by enzymatic processes at the cell membrane, and indirectly, with microbially produced H2S acting as the reductant.(ABSTRACT TRUNCATED AT 400 WORDS)

Environmental biochemistry of arsenic.

Microorganisms are involved in the redistribution and global cycling of arsenic. Arsenic can accumulate and can be subject to various biotransformations including reduction, oxidation, and methylation. Bacterial methylation of inorganic arsenic is coupled to the methane biosynthetic pathway in methanogenic bacteria under anaerobic conditions and may be a mechanism for arsenic detoxification. The pathway proceeds by reduction of arsenate to arsenite followed by methylation to dimethylarsine. Fungi are also able to transform inorganic and organic arsenic compounds into volatile methylarsines. The pathway proceeds aerobically by arsenate reduction to arsenite followed by several methylation steps producing trimethylarsine. Volatile arsine gases are very toxic to mammals because they destroy red blood cells (LD50 in rats; 3.0 mg kg-1). Further studies are needed on dimethylarsine and trimethylarsine toxicity tests through inhalation of target animals. Marine algae transform arsenate into non-volatile methylated arsenic compounds (methanearsonic and dimethylarsinic acids) in seawater. This is considered to be a beneficial step not only to the primary producers, but also to the higher trophic levels, since non-volatile methylated arsenic is much less toxic to marine invertebrates. Freshwater algae like marine algae synthesize lipid-soluble arsenic compounds and do not produce volatile methylarsines. Aquatic plants also synthesize similar lipid-soluble arsenic compounds. In terrestrial plants, arsenate is preferentially taken up 3 to 4 times the rate of arsenite. In the presence of phosphate, arsenate uptake is inhibited while in the presence of arsenate, phosphate uptake is only slightly inhibited. There is a competitive interaction between arsenate and phosphate for the same uptake system in terrestrial plants. The mode of toxicity of arsenate is to partially block protein synthesis and interfere with protein phosphorylation but the presence of phosphate prevents this mode of action. There appears to be a higher affinity for phosphate than arsenate with a discriminate ratio of 4:1. It is estimated that as much as 210 x 10(5) kg of arsenic is lost to the atmosphere in the vapor state annually from the land surface. The continental vapor flux is about 8 times that of the continental dust flux indicating that the biogenic contribution may play a significant role in cycling of arsenic. It has not been established whether volatile arsenic can be released by plants. Further studies are needed to determine mass balances in the rate of transfer (fluxes) of arsenic in the environment.

Determination of saccharides in biological materials by high-performance anion-exchange chromatography with pulsed amperometric detection.

High-performance anion-exchange chromatography (HPAEC) coupled with pulsed amperometric detection (PAD) under alkaline conditions (pH 9-13) separates aminosaccharides, neutral saccharides and glycuronic acids based upon their molecular size, saccharide composition and glycosidic linkages. Carbohydrates were extracted by utilizing 0.5 M H2SO4 (neutral monosaccharides), 0.25 M H2SO4 coupled with enzyme catalysis (glycuronic acids) and 3 M H2SO4 (aminosaccharides). Solid-phase extraction with strong cation and strong anion resins was used to partition the cationic aminosaccharides and anionic glycuronic acids and to deionize acid extracts for neutral saccharides. Separation was conducted on a medium-capacity anion-exchange column (36 mequiv.) utilizing sodium hydroxide (5-200 mM and sodium acetate (0-250 mM) as the mobile phase. The saccharides were detected by oxidation at a gold working electrode with triple-pulsed amperometry. HPAEC-PAD was found superior to high-performance liquid chromatography with refractive index (RI) detection for neutral monosaccharides and aminosaccharides and to low-wavelength UV detection for glycuronic acids in terms of resolution and sensitivity. HPAEC-PAD was not subject to interferences as was the case for low UV detection (210 nm) or RI analyses and was highly selective for mono- and aminosaccharides and glycuronic acids. The use of HPAEC-PAD was applied for the determination of the saccharide composition of organic materials (plant residues, animal wastes and sewage sludge), microbial polymers and soil.

Evolution of trimethylarsine by a Penicillium sp. isolated from agricultural evaporation pond water.

Arsenicals are used in agriculture as pesticides and defoliants. In the Central Valley of California, arsenic is present in soil at naturally high concentrations, being derived from marine sedimentary parent material of the Coastal Range. Due to intense agricultural irrigation, soluble arsenic is leached from the soil and accumulates in evaporation ponds where it may pose an environmental threat to the waterfowl and wildlife. A Penicillium sp. isolated from evaporation pond water was found to be capable of methylating and subsequently volatilizing organic arsenic. The major focus of this study was to characterize the environmental conditions, including culture media, arsenic substrates, pH, temperature, and the presence of phosphates, carbohydrates and amino acids on the methylation of arsenic. Trimethylarsine was monitored by gas chromatography (GC)-flame ionization detection and identified by GC-mass spectrometry. The conditions or additions for optimum trimethylarsine production were: a minimal medium in which 100 mgl-1 methylarsonic acid served as the arsenic source, pH 5-6, temperature of incubation 20 degrees C, and phosphate concentration of 0.1-50 mM (KH2PO4). The addition of carbohydrates and sugar acids to the minimal medium suppressed trimethylarsine production. The amino acids phenylalanine, isoleucine, and glutamine promoted trimethylarsine production with an enhancement ranging from 10.2- to 11.6-fold over the control without amino acid supplementation. The information obtained from this study may be useful in developing a bioremediation approach in trapping the arsenic gas evolved from soil or water as a mitigation alternative in the cleanup of arsenic contamination.

Determination of aminosaccharides by high-performance anion-exchange chromatography with pulsed amperometric detection.

High-performance anion-exchange chromatography (HPAC) was used for the determination of aminosaccharides in microbial polymers, chitin, animal waste, sewage sludge, plant residues and soil. The aminosaccharides, galactosamine, mannosamine and glucosamine were separated on a strong anion-exchange column with 1OmM sodium hydroxide as the eluent and determined by pulsed amperometric detection (PAD). The HPAC-PAD methodology was compared with high-performance liquid chromatography (HPLC) with refractive index detection (RI) in terms of selectivity and sensitivity for aminosaccharides. The results indicate that HPAC-PAD required less sample preparation, and was more precise and nearly two orders of magnitude more sensitive than HPLC-RI. HPAC-PAD was not subject to matrix interferences and was highly selective for aminosaccharides. More than 3% of the total nitrogen in alfalfa, and 20% of that in straw, was found to be present as aminosaccharides.

Volatilization of selenium from agricultural evaporation pond sediments.

Microbial volatilization of Se was evaluated as a means of detoxifying Se-contaminated sediments. Sediment samples containing 60.7 (Kesterson Reservoir) and 9.0 mg Se kg-1 (Peck ponds) were incubated for 273 days in closed systems located in the greenhouse. Volatile Se was collected from a continuous air-exchange stream using activated carbon. Various economical and readily available organic and inorganic amendments were tested for their capacity to enhance the microbial process, including Citrus (orange) peel, Vitis (grape) pomace, feedlot manure, barley straw, chitin, pectin, ZnSO4, (NH4)2SO4, and an inoculum of Acremonium falciforme (an active Se methylating fungus). With the Kesterson sediment, the highest Se removal (44.0%) resulted from the combined application of citrus peel and ZnSO4, followed by citrus peal alone (39.6%), and citrus peel combined with ZnSO4, (NH4)2SO4 and A. falciforme (30.1%). Manure (19.5%), pectin (16.4%), chitin (9.8%) and straw plus N (8.8%) had less pronounced effects. Without the amendments, cumulative Se volatilization was 6.1% of the initial inventory. Grape pomace (3.0%) inhibited the process. With the Peck sediment, the highest amount of Se removed was observed with chitin (28.6%), manure (28.5%), and citrus peel alone (27.3%). Without amendments, 14.0% of the native Se was volatilized in 273 days. Cumulative Se volatilization was 24.7% with citrus plus Zn and N, 17.2% with citrus plus Zn, and 18.8% with citrus plus Zn, N and A. falciforme. Pectin (15.2%), straw plus N (16.4%), and grape pomace (7.3%) were among the less effective amendments for the Peck sediment. The differences in the effectiveness of each treatment between the two seleniferous soils may be a result of the residual N content of the sediments. With the Kesterson sediment, which was high in organic C and N, added N inhibited volatilization of Se, while with Peck sediments (low in organic C and N) N-rich materials tended to accelerate Se volatilization. Inoculation with A. falciforme did not enhance Se evolution from either sediment, indicating that there was a sufficient population of microflora capable of producing gaseous Se.